Historically, naval and commercial watercrafts were typically operated in the speed ranges of 10 to 30 knots plus. Because of developments in hydrodynamic theories of ship resistance and hull form design, ships that travel at speeds greater than 30 knots are now available. Based on this technology, the Navy has been developing high speed ships with sprint/transient speeds of 38 to 45 knots. The private sector is also actively pursuing the development of high-speed ships such as fast ferryboats that can travel at about 40 to 50 knots. Along with the increased capacity for speed comes the demand for efficient propulsors for high-speed ships.
For good propeller performance, conventional propellers are designed to operate without blade surface cavitation. This type of propeller is termed a sub-cavitating propeller. A typical blade section 100 for sub-cavitating propellers is shown in FIG. 1A. As shown, the blade section 100 is substantially streamlined from the leading end 110 to the trailing end 120. Operating in a ship wake with an inclined shaft, the blade surfaces of these propellers typically start to experience surface cavitation between 25 and 29 knots. Increasing the ship speed by more than 5 knots above the surface cavitation inception speed typically results in severe propeller cavitation. Severe cavitation typically results in the loss of propeller efficiency, erosion, and thrust breakdown.
As shown in FIG. 1A, marine propellers have historically utilized blade sections with airfoil shapes having known cavitation characteristics. At high speeds, these sections will begin to cavitate either at their leading edge due to angle of attack fluctuations, or in the middle of the upper surface due to the low pressure. As blade surface cavitation grows with increasing speed, it eventually covers the entire upper side of the blade. When this occurs, the blade is considered to be operating in the super-cavitating condition. The upper surface of the blade section is covered by a vapor or ventilated air cavity starting at the leading edge and extending to and beyond the trailing edge. In that condition, the hydrodynamic loading is totally controlled by the pressure surface blade profile. Unfortunately, the shape of a pressure face designed for sub-cavitating operation is usually not effective for producing lift in the full cavitating mode.
A super-cavitating foil typically has a sharp leading edge, where surface cavitation is intentionally initiated. A sample super-cavitating blade section 150 is shown in FIG. 1B. As illustrated, the super-cavitating blade section 150 has a sharp edge at the leading end 160 and a blunt edge at the trailing end 170. The hydrodynamic efficiency (lift-to-drag ratio) of a foil operating in a super-cavitating mode is governed by the lower surface camber. However, when super-cavitating foils are used in sub-cavitating conditions, the blunt trailing edge of the section produces a significant separated flow which results in high drag.
Consequently, it is desired to have a foil that operates effectively in both sub-cavitating and super-cavitating flow regimes. U.S. Pat. No. 5,551,369 discloses a duel-cavitating foil. However, U.S. Pat. No. 5,551,369 is directed towards hydrofoils, which can be controlled mechanically by directly changing the angle of the foils or by using flaps. Without a controllable pitch mechanism, this is not a viable option for propeller systems.
Additionally, propellers are now used to produce negative thrust to slow down and stop watercrafts. Two methods are currently used to achieve negative thrusts. One method is to use a controllable pitch device to rotate the propeller pitch to generate negative thrust. The challenge of using this method is that it is costly to fabricate, it requires a large space to house the controllable pitch mechanical device, and it is a maintenance challenge.
Another method is to reverse the propeller's rotational direction. With the recent advance in electric motor technology, the polarity of electric current can be easily switched to reverse propeller shaft and RPM direction to generate large negative thrust for emergency stopping. However, with conventional super-cavitating propellers, when a propeller RPM is operated in a reverse direction, the flow reverses and flows from the trailing edge toward the leading edge. As shown in FIG. 1B, the trailing end of conventional super-cavitating foils are blunt, which produces significant flow separation. Consequently, it is desired to have a propeller foil that operates effectively in a super-cavitating flow regime and is able to produce emergency stopping.